Apply COPY vs ADD best practices

Should only use `ADD` for URLs (and even then, `curl` or `wget` are preferred, for numerous reasons, even within this repo) and when extracting archive contents as part of the `ADD` operation; otherwise `COPY` is clearer and (slightly) more efficient.

hhvm/Dockerfile

@@ -15,7 +15,7 @@ RUN apt-get update -y \
 
 RUN mkdir -p /var/www
 
-ADD server.ini /etc/hhvm/server.ini
+COPY server.ini /etc/hhvm/server.ini
 
 RUN usermod -u 1000 www-data